In the case of current semiconductor devices (chips), there is the risk that information stored in them can be read out by unauthorized persons, by the operating mode of the chips being analyzed.
A large number of protective measures intended to protect the chips against unauthorized access are therefore already in existence.
For example, there are shielding measures, security-enhancing sensors (light sensors, glitch sensors, etc.) or special encapsulations of the chip, which are intended to prevent its analysis.
A further possible way of protecting the chips against unauthorized access is offered by what is known as “face-to-face” (F2F) technology.
In the case of F2F technology, the chips are made to face one another in such a way that the respective surfaces with the functional elements of the chip contained in them are arranged on one another. The surfaces are connected to one another in an electrically conductive manner, for example by soldering or adhesive bonding, by means of special contact pads.
In the case of a protected chip stack, this connection of the chip surfaces is formed in such a way that the two chips lying one on top of the other can only be separated with great effort. Attempts to separate them by increasing the temperature or mechanically shearing them apart lead to destruction of the chips before the connection that is formed is released.
Therefore, a protected chip stack is to be understood as meaning a stack of chips which is intended to prevent sensitive information contained in it from being found out by means of a secure connection between the stacked chips.
Nevertheless, in the case of the current protected chip stacks there is still the possibility of separating the two connected chips by means of chemical attack without destroying them and consequently, by analysis of the chip, to read out information contained in it.
Such an attack can be averted by cryptographic protocols. However, this requires the use of encryption algorithms, such as for example a block cipher (for example DES, AES) or a stream cipher. In many products, the implementation of such complex algorithms is out of the question, either for reasons of cost or for reasons of efficiency.